1. Field of the Invention
The present invention relates to a pillbox vacuum window for use in a microwave tube as input/output windows, and a method of manufacturing the pillbox vacuum window.
2. Description of the Related Art
FIG. 1 is a cross-sectional view illustrating the structure of a general microwave tube.
Referring to FIG. 1, the microwave tube comprises electron gun 31 for emitting electron beams, high frequency circuit 32 for interacting an RF signal (microwave) received thereby with an electron beam emitted from electron gun 31 to amplify and deliver the resulting electron beam, collector 33 for collecting the electron beam which has passed through high frequency circuit 32, and anode electrode 34 for guiding the electron beam emitted from electron gun 31 into high frequency circuit 32.
The microwave tube also comprises pillbox vacuum windows 35, 36 as input/output windows of the RF signal for two purposes, i.e., for reducing the RF signal loss and for sealing the microwave tube in a vacuum.
Techniques related to the pillbox vacuum windows are disclosed, for example, in JP-A-04-092341 and JP-A-08-154001.
The following description will be focused on the configuration of a conventional pillbox vacuum window.
FIG. 2A is a longitudinal sectional view illustrating an exemplary configuration of a conventional pillbox window, and FIG. 2B is a cross-sectional view illustrating an exemplary configuration of the conventional pillbox vacuum window.
Referring to FIG. 2A, the illustrated exemplary conventional pillbox vacuum window comprises ceramic disk 41, metallization layer 42, and metal parts 43, 44.
Ceramic disk 41 is positioned at the center of the pillbox vacuum window.
Metallization layer 42 is formed on the peripheral side surface of ceramic disk 41 and on the peripheral areas of both plane surfaces of the same.
Metal parts 43, 44 are bonded to ceramic disk 41 by brazing through metallization layer 42 so as to sandwich ceramic disk 41 therebetween from both plane surfaces of ceramic disk 41.
Metallization layer 42 is required to have a length equal to or larger than a leak path in order to prevent a leak between ceramic disk 41 and metal parts 43, 44.
The leak path refers to the length of portions of ceramic disk 41 and metal parts 43, 44 which are bonded to each other through metallization layer 42.
In this conventional example, the leak path has radial length L of 0.5 mm on both plane surfaces of ceramic disk 41. Accordingly, metallization layers 42 on both plane surfaces of ceramic disk 41 also have a radial length of 0.5 mm or larger.
In this conventional example, ceramic disk 41 has thickness t of 0.2 mm, cylindrical waveguide pipe 45 has a cylindrical cavity, the inner diameter R (FIG. 2B) of which is 4 mm, and square waveguide pipes 46a, 46b (FIG. 2A) each have a long side a (FIG. 2B) of 7.11 mm and a short side b (FIG. 2B) of 3.56 mm.
FIG. 3A is a longitudinal sectional view illustrating another exemplary configuration of a conventional pillbox vacuum window, and FIG. 3B is a cross-sectional view illustrating another exemplary configuration of the exemplary conventional pillbox vacuum window.
Referring to FIG. 3A, the pillbox vacuum window of this conventional example comprises ceramic disk 51, metallization layer 52, and metal parts 53, 54, 55.
Ceramic disk 51 is positioned at the center of the pillbox vacuum window.
Metallization layer 52 is formed on the peripheral side surface of ceramic disk 51.
Metal part 55 is bonded to ceramic disk 51 by brazing through metallization layer 52 to sandwich ceramic disk 51 from the peripheral side surface of ceramic disk 51.
Metal parts 53, 54 are bonded to metal part 55 by brazing to sandwich metal part 55 therebetween from both plane surfaces of ceramic disk 51.
In this conventional example, ceramic disk 51 has thickness t of 0.4 mm. Accordingly, a leak path also has a length of 0.4 mm in the thickness direction, and metallization layer 52 also has a length of 0.4 mm in the thickness direction on the peripheral side surface of ceramic disk 51.
In this conventional example, cylindrical waveguide 56 has a cylindrical cavity, the inner diameter R (FIG. 3B) of which is 4 mm, while square waveguides 57a, 57b (FIG. 3A) each have a long side a (FIG. 3B) of 7.11 mm, and a short side b (FIG. 3B) of 3.56 mm.
FIG. 4 is a graph for describing the voltage standing wave ratio (hereinafter called “VSWR”) vs. frequency (in GHz) of the conventional pillbox vacuum window illustrated in FIGS. 2A and 2B.
In the conventional pillbox vacuum window illustrated in FIGS. 2A and 2B, metallization layer 42 is required to have a radial length of 0.5 mm or larger on both plane surfaces of ceramic disk 41. As metallization layer 42 has a larger length in this way, resonance occurs in a peripheral area of ceramic disk 41 at around 37 GHz within an available frequency band (26.5 to 40.0 GHz) of the pillbox vacuum window, giving rise to an abrupt rise of VSWR, as shown in FIG. 4.
Also, in the conventional pillbox vacuum window illustrated in FIGS. 2A and 2B, ceramic disk 41 must be increased in outer diameter more than is necessary, together with metallization layer 42 which is formed to have a length of 0.5 mm or more. Ceramic disk 41 is made of a dielectric material such as alumina, beryllia or the like. Accordingly, the larger outer diameter of ceramic disk 41 results in a higher proportion of dielectric material which occupies the overall pillbox vacuum window, causing the dielectric material to exert a larger influence on the VSWR characteristics. Consequently, as shown in FIG. 4, VSWR is increased due to the influence of the dielectric material at around 35 GHz within the available frequency band of the pillbox vacuum window.
Likewise, in the conventional pillbox vacuum window illustrated in FIGS. 3A and 3B, ceramic disk 51 is required to have a thickness of 0.4 mm, so that ceramic disk 51 must be made thicker than is necessary. Also, a larger thickness of ceramic disk 51 results in a larger length of metallization layer 52 in the thickness direction. Accordingly, the VSWR characteristics are similar to those shown in FIG. 4 in that resonance occurs within an available frequency band to increase VSWR.
Also, the conventional pillbox vacuum window illustrated in FIGS. 3A and 3B has the problem that it is difficult to maintain the dimensional accuracy for each part during brazing.